Preclinical molecular imaging is used in studying animal models of human disease, evaluating new therapeutic drugs, and in carrying out research in basic biology. Optical imaging techniques, such as bioluminescence and multispectral fluorescence imaging, currently are widely used for preclinical functional imaging despite their depth dependent limitations on quantitation, resolution and sensitivity. Nuclear imaging techniques, SPECT and PET, are more closely translational. Dedicated preclinical SPECT and PET systems provide quite good imaging capability, but can be expensive, difficult (for non-experts), and slow to use. Optical systems are widely available, relatively affordable, easy to operate, and quick (for setup, imaging, and data analysis; i.e. high-throughput). For some imaging tasks a dedicated SPECT system would be essential, but for many studies a high-throughput acquisition of projection images would be sufficient and gains from cost reduction and speed could be significant. High resolution planar images of gamma-ray emitting radiotracers can be obtained by retrofitting a small animal optical imaging system (such as an IVIS Spectrum (Perkin-Elmer)) with a pinhole collimator and a large area CsI:Tl scintillator with a novel morphology. Recent progress at Radiation Monitoring Devices (RMD) has led to thick, transparent, crystalline microcolumnar structure (CMS) scintillator detectors of CsI(Tl) that simultaneously provide high gamma-ray absorption efficiency, high intrinsic spatial resolution, and bright light emission. The ability to use an existing commercial preclinical optical imaging system to rapidly acquire planar gamma ray images with good spatial resolution of one or possibly multiple animals would be a new and useful tool for high throughput screening of molecular imaging probes. Adding a nuclear imaging capability to the many existing preclinical optical imaging systems is an appealing opportunity for greatly expanded access to that modality of molecular imaging. The goal of this proposal is to build and characterize an insert which can be placed in an existing optical imaging system, where the insert uses a collimator and scintillator to enable gamma rays to be imaged by the existing sensitive CCD camera. The collimators used will be from a commercial SPECT imaging system. In this project we will: (1) develop protocols for production of large (15 cm diameter) thick (up to 4 mm) CMS CsI(Tl) films with improved light transmission; (2) simulate collimator and detector configurations using the GATE software package; (3) produce three large CMS detectors for use in an optical system insert; (4) fabricate an insert with flexible geometry for a variety of collimator and scintillator configurations, and perform phantom imaging studies to characterize the system; and (5) conduct two proof-of-concept animal imaging studies to demonstrate the potential for dynamic imaging and for high resolution imaging of a smaller field of view.